Identify unnecessary inputs of NN
An interesting way to check for redundancy not only in the first layer but in any layer is to see the weights of you neuron. Let's consider the input layer: The nodes that are connected with the weights that have high values are the most important for your NN performance and in other words your input to these nodes in what mostly makes sense for your task. 

If you are in Keras, you can use the get_weights method(). You may have a look here.